Homing of B lymphocytes into specialized microenvironments within secondary lymphoid tissues is essential for normal immune function, yet the molecular cues guiding this cellular traffic are not well defined. Evidence suggests the involvement of chemokines (1-5), but no chemokine has been shown to have the required expression pattern or chemoattractant activity (6). Here we describe a chemokine, B Lymphocyte Chemoattractant (BLC), that is highly expressed in the follicles of Peyer's patches, spleen and lymph nodes. BLC strongly attracts B lymphocytes while promoting migration of only small numbers of T cells and macrophages and therefore is the first chemokine identified with selectivity for B cells. Recently an orphan chemokine receptor, Burkitt's Lymphoma Receptor 1 (BLR1) was found to be required for B cell migration into lymphoid follicles (4). We also disclose that BLC stimulates calcium influx and chemotaxis in cells transfected with BLR1, indicating that BLC functions as a BLR1 ligand and guides B lymphocytes to follicles in secondary lymphoid organs. BLR1:BLC interactions provide a valuable target for phamaceutical development and therapeutic intervention.